1. Field of Technology
This invention relates generally to a soldering method and apparatus that uses a brazing material to bond components together, and more particularly to a soldering method and an apparatus which may be suitable for use in soldering applications performed in small areas or requiring fine detailed work, such as in bonding electronic components to a printed circuit board.
2. Background of Technology
Traditionally, the soldering methods most suitable for industrial mass production have been the flow method and the reflow method. For example, the flow method is often used for mounting components onto a printed circuit board or other workpiece (hereinafter referred to generally as "substrate"). In the flow method, a substrate on which electrical or other components have been placed is coated with flux, and then soldering is achieved by passing the substrate, components and flux through a molten solder bath thus soldering the components to the substrate.
In the reflow soldering method, a mixture of flux and soldering paste is applied, or printed onto that part of the substrate on which the electrical or other components are to be soldered. Then, the components are placed on the substrate over this flux and solder paste layer and soldering is achieved by passing the substrate and components through a heating oven, otherwise known as a "reflow oven" thus soldering the components to the substrate.
However, it is often difficult to consistently and uniformly produce an even coating of flux on the bonding portions of the substrate. This is true whether the flux coating is applied over components, as in the flow method, or directly onto the substrate, as in the reflow method. For this reason, the coating step has become a major obstacle to achieving automation in semiconductor manufacture as substrate patterns have become finer and the density of electrical or other components has increased. Furthermore, in both methods, residual flux on the substrate may be corrosive or otherwise detrimental to the electrical or other components, and thus, it is usually necessary to rinse and clean the substrate after soldering has been completed, thereby requiring yet another process step. Additionally, when soldering a TAB substrate to a liquid crystal panel, for example, there is the potential problem that the soldering flux may splash and contaminate the polarizing film. Consequently, it is usually necessary to attach a protective film to the polarizing film before soldering, and then to remove this film after the soldering is finished. These additional process steps increase the total number of process steps, time, and labor, resulting in increased manufacturing costs and lower manufacturing efficiency.
Known methods of removing or otherwise cleaning organic substances, such as flux from a substrate include the wet rinse method which uses an organic solvent to remove organic substances such as flux, and the dry rinse method, which removes organic substances such as flux by causing decomposition by chemical reaction, for example by irradiation with ozone, and/or ultraviolet light.
However, the wet rinse method requires an additional cleaning process for removing the rinsing agent after the organic substances have been rinsed off, a process for drying the substrate, and a facility for performing these processes. These additional process step requirements for the wet rinse method necessitate a massive amount of time and labor and typically result in increased manufacturing costs and lowered overall efficiency.
On the other hand, thorough cleaning of organic substances cannot always be expected using the dry rinse method since it is difficult to completely remove certain organic substances, especially those possessing, for example, especially large molecular weight.
To avoid the problems of conventionally used rinse methods, no-rinse fluxes containing very little or no activators, for example, chlorine, have been in use recently. However, because these fluxes possess poor wettability compared with conventional fluxes, the use of these no-rinse fluxes raises the potential that the resulting bond may be incomplete or may have insufficient bonding strength to be used reliably for joining the components to a substrate.
Furthermore, when components must be soldered on both sides of a substrate, the heat treatment applied during the soldering of components on one side often causes the formation of an oxide film, for example, CuO on the surface of the copper pads and electrodes. The heating also may cause the loss of even the initial level of wettability on the side still to be bonded. In some cases, the heat treatment even leads to increased contact resistance during soldering.
Unexamined Japanese patent application No. H03-174972 discusses a method which obtains excellent adhesion of a soldering material to a substrate. In this method, the substrate bonding surface is exposed to argon plasma generated in a low pressure argon gas atmosphere by an electrical discharge. This argon plasma generated by electrical discharge is used to remove impurities from the bonding surface of the substrate and to improve its wettability. Then the substrate is coated with the soldering material.
However, this method necessarily requires a vacuum pump and a vacuum chamber for obtaining the low pressure argon gas atmosphere, making the overall processing apparatus large and complicated. Furthermore, argon, the gas used for generating electrical discharge, does improve the wettability between the soldering paste and the substrate, thus improving the printability of the paste solder, however, it does not improve the wettability of the solder itself for the components since it does not remove the oxide film. Therefore, using this method, it is still necessary to coat the soldering surface with a flux in order to remove the oxide film before soldering, and consequently, this flux must be rinsed off after the soldering process is completed.
Furthermore, unexamined Japanese patent application No. S3-127965 discusses an apparatus that is equipped with a device which generates an arc discharge between itself and the components to be soldered, and thus quickly heats the components. This device makes it easy to solder a component, even if its heat capacity is excessively large. However, with this device, the electrical discharge is used to generate the heat required for soldering, and cannot be used to remove an oxide film or improve the wettability. Furthermore, since the gas used for generating the discharge is an inert gas, removal of the oxide film would not be expected to occur in theory. This method also requires that the soldering surface be coated with a flux in order to remove the oxide film, necessitating that the flux must then be rinsed off after soldering. Thus, additional process steps are required which lower the efficiency and raise the costs of the overall manufacturing process.
Accordingly, it is desired to provide an improved method and apparatus for bonding using brazing material.